1. Field of the Invention
This invention relates to a filtering medium used for precision filtration such as a filter for liquid used for washing electronic devices or a prefilter for liquid or gas used for producing medicines. More particularly, it relates to a filtering medium for precision filtration, obtained by hot-melt-adhering a non-woven fabric composed of microfine fibers having the intersections thereof hot-melt-adhered, to a net made of hot-melt adhesive fibers, which medium has no change in the pore diameter due to heating and a superior processability for pleat form, etc., and a process for producing the above filtering medium.
2. Description of the Related Art
In recent years, industries directed to electronics, biochemicals, etc. have been developed, and an opportunity using a purified gas or liquid for producing these materials have been increasing. Heretofore, as a filtering medium for a precision filtration, glass fiber non-woven fabrics, synthetic fiber non-woven fabrics composing of microfine fibers, etc. have been used. However, the above glass fiber non-woven fabrics have such problems that the fabrics are weak to alkali-resistance, and when they are processed into a pleat form in order to afford a large surface area for filtration or processed into various steric shapes, they are inferior in the so-called shapability, and so on. On the other hand, the above synthetic fiber non-woven fabrics have such advantages that they have a less specific gravity and a lighter weight as compared with glass fiber non-woven fabrics; the shapability is better than that of glass fiber non-woven fabrics; they are cheaper; the material does not scatter away at the time of processing the filtering medium as in the case of glass fibers so that they are easily dealt with; and so on. Thus, the filtering medium of the synthetic fiber non-woven fabrics has rapidly come to be used broadly.
For the filtering medium of synthetic non-woven fabrics, polyester spun bonded non-woven fabrics, polypropylene melt-blown non-woven fabrics, etc. have been used, but there is a problem that mesh-openings of the non-woven fabric enlarge due to heating, vibration, abrasion, etc. to make the pore size larger, thus the stability of the pore size is inferior.
As a material having improved the shapability of the filtering medium of non-woven fabric, a filtering medium obtained by hot-melt-adhering a non-woven fabric onto a net-form sheet has been known. Japanese patent application laid-open No. Hei 1-194912 discloses a filter obtained by hot-melt-adhering a microfine fiber non-woven fabric brought into an electret, onto a net-form material, and Japanese patent application laid-open No. Hei 4-346805 discloses a filtering medium obtained by hot-melt-adhering a microfine fiber non-woven fabric onto a net composed of hot-melt-adhesive filaments and metal-gage wires.
In the case of any of the above products obtained by hot-melt-adhering a non-woven fabric onto a net-form sheet, regular fibers such as those of melt-blown polypropylene, melt-blown polyesters, etc. have been used. That is, a product obtained by hot-melt-adhering the microfine fiber web so that the intersections of the fibers have not been hot-melt-adhered, a product obtained by hot-melt-adhering the microfine fiber web so that the intersections of the fibers have been partly hot-melt-adhered by means of emboss roll, calender roll or the like, a product obtained by laminating a net onto a web or a hot-press-adhered non-woven fabric, followed by hot-melt-adhering the non-woven fabric onto a net by means of a heating means such as calender roll, dryer or the like.
However, when the hot-melt-adhesion of the non-woven fabric is observed from the micro-view point, it is difficult for the above filtering medium of non-woven fabrics to sufficiently hot-melt-adhered through the intersections of the fibers without damaging the air permeation resistance of the medium. For example, in the case of emboss roll, parts other than hot-pressed parts have not been hot-melt-adhered. In the case of calender roll, the upper and under surfaces of the non-woven fabric have been mostly hot-melt-adhered, but the hot-melt-adhered parts are few at the central parts in the direction of the thickness, or even if there are many hot-melt-adhered parts, such parts are liable to be weak. In the case of such non-woven fabrics having different hot-melt-adhesion states of fibers as described above, mesh-openings in the non-woven fabric are liable to be enlarged on account of heating at the time of processing the woven fabric into pleat weave or into a cylindrical form, heat-sealing both the end surfaces of a cylinder with an end surface member of a synthetic resin, or heating a filtering medium for heat-sterilization, or on account of vibration, impulse, vibration of housing at the time of filtration, etc. Thus, there is a drawback that the maximum pore size of the non-woven fabric becomes larger, that is, a drawback that the pore size stability is inferior. In particular, a product having a basis weight as high as about 25 g/m.sup.2 or more is liable to be insufficient in the hot-melt-adhesion of fibers in the vicinity of the intermediate part in the thickness direction, of non-woven fabric, and is inferior in the pore size stability, and often had a maximum pore size after heated, increased as large as by 25% or more.
Further, the filtering medium obtained by hot-melt-adhesion at a high temperature and under a high pressure according to calender roll process or the like is somewhat improved in the pore size stability, but since the whole of the fibers is melted and changed into a film form, there is raised a problem that the air permeation resistance becomes notably high.